Sign the assembly with a strong name

[flensrocker]

• Updated RichardSzalay.MockHttp to 6.0.0 (it's strong name signed)
• Updated .Net 4.5 to .Net 4.5.2 (otherwise I can't compile with VS 2022)
• Added .NET 6.0

To sign an assembly with a strong name you have to create a snk-file with:
sn -k WebPush.snk

For the InternalsVisibleTo attribute you also have to sign the test assembly. You can use the same key file.

To extract the public key from the snk-file:

sn -p WebPush.snk pub.snk
sn -tp pub.snk
(copy key)
del pub.snk

See #30